We study metallic nanoparticles arranged in chiral structures, with the aim of enhancing the inherently weak chiral light-matter interaction, whether in absorption, fluorescence, or Raman optical activity. Our goal is to gain an improved physical understanding of how light interacts with chiral structures at the nanoscale, and conversely, how nanostructures can shape the polarisation of local electromagnetic fields. This research combines spectroscopic experiments and numerical modelling to explore a broad range of fundamental questions on novel chiroptical effects, as well as direct the development of future applications in molecular detection. [10], [11], [12]. This project is funded by a 5-year Rutherford Discovery Fellowship from the Royal Society of New Zealand.

Tamm plasmons are electromagnetic modes confined at the interface between a noble metal layer and a multi-layer (Bragg) dielectric mirror. They provide a simple and versatile one-dimensional platform for enhanced light-matter interaction, allowing excitation at normal incidence, and a high quality factor. We have studied the conditions for critical coupling of light to Tamm plasmons yielding zero reflection and complete absorption, and provided the first proof-of-principle sensing application for Tamm plasmons, using mesoporous multilayers. [23], [24]

Surface plasmon resonance has found applications in optical sensing, where it is routinely used to monitor minute changes of refractive index near the surface of a gold layer. This forms the basis of a mature and reliable sensing platform, however it can suffer from a lack of specificity: the signals are blind to the nature of an event, e.g. what type of molecule binds to or modifies the surface. We have designed an original excitation scheme through a microscope objective to allow the simultaneous measurement of surface plasmon resonance shifts in the Kretschmann configuration, and the collection of surface-enhanced Raman scattering spectra from molecules bound to the gold surface. [16]

When nanoparticles arranged in a 2-dimensional array are illuminated with a wavelength commensurate to the inter-particle separation, a coherent interaction can develop that electromagnetically couples a large number of particles. In ordered arrays, such coupling gives rise to a diffraction effect that can strongly affect the plasmon resonance of the particles. In particular a sharp spectral peak can be observed in transmission, that depends on both the single particle response and the geometrical arrangement of the particles. [2], [5], [8]

For nanoparticles and molecules, with sizes much smaller than the light wavelength, the coupled-dipole approximation (CDA) provides a powerful and physically intuitive technique, with a broad range of current applications in nano-optics and spectroscopy. We have developed computer programs implementing the CDA to describe plasmonic optical activity in helical clusters of gold nanoparticles. We have also used the CDA to model the optical response of dye molecules adsorbed onto nanoparticles, in order to understand the role of dye-dye interactions at large concentrations, and the subtle effects of dye-nanoparticle interaction. [10], [28]

In close collaboration with the Raman lab led by Prof Eric Le Ru we also pursue new ideas in the areas of Surface-Enhanced Raman Scattering (SERS), modified molecular absorbance, Mie and T-matrix theory for electromagnetic scattering. [29]